1. Field of the Invention
The present invention relates to a method for recovering cadmium from cadmium-containing scrap metal. In particular, the present invention relates to a method for recovering cadmium from scrap metal and converting it to a nonleachable form.
2. Discussion of Background
Cadmium is a rare metal, roughly as plentiful as silver in the Earth's crust. It is found chiefly as an impurity in zinc ores, although one primary cadmium mineral, greenockite (cadmium sulfide, CdS) occurs in some hydrothermal ore bodies. Cadmium has physical and chemical properties intermediate between those of zinc and mercury.
Cadmium's unusual properties have historically made it a valuable ingredient in metallurgy. Low-melting solders, and fusible metals used in fire and steam safety, frequently contain cadmium; a well-known example is "Wood's metal," an alloy made by combining one part each of tin and cadmium with two pans lead and four parts bismuth. Cadmium is used in coating iron and steel to protect them from corrosion; it functions in this role much like zinc on galvanized iron, but is far more effective on a weight-for-weight basis. Cadmium is also used in the contacts of electrical relays meant to handle high currents--any relay rated for ten amperes or more likely has cadmium-bearing contacts.
The ready oxidizability and reducibility of cadmium (as evidenced by its low reduction potential of -0.4026 V) makes it useful in energy storage, notably in rechargeable nickel-cadmium cells. Cadmium is also used as an additive to improve the performance of lead-acid storage cells, especially in restoring old cells which have lost some of their capacity.
An additional property of cadmium--or more accurately of its isotope cadmium-113 which makes up 12.2% of naturally-occurring cadmium--is that it has an unusually high absorption cross-section for thermal neutrons. As a result, cadmium metal is widely used in nuclear reactor control and safety rods, housings and shielding assemblies for thermal-neutron sources and measuring equipment, and various other devices used throughout the nuclear industry.
Because of these multiple uses, cadmium is present to some degree in scrap metal from a wide variety of sources.
Cadmium is readily absorbed by the human body from food or water, but is much less readily excreted, and so becomes concentrated to a high degree in some body tissues. Like mercury, cadmium is highly toxic to humans, typically affecting the kidneys, bones, liver and nervous system. Inhaling fumes of the metal or freshly-formed oxide can cause lung scarring, pneumonia, emphysema, or even death. As a result, it is listed as a characteristic hazardous waste with a T.C.L.P. limit of only one part per million. This means that to be considered as nonhazardous waste, any cadmium-beating waste must be in such a form that, after leaching by simulated groundwater in an EPA-specified laboratory procedure, the water will contain less than one part per million of dissolved cadmium.
The volatility of cadmium, combined with its toxicity, makes cadmium-bearing scrap difficult and often impractical to reprocess. Nearly all scrap metal reclamation schemes use heat to bum off grease, paint and other contaminants. Heat may also be applied to melt the metal into more compact, easily-handled forms such as ingots, or disperse it into granules for more efficient chemical processing. When cadmium-bearing scrap is heated, the cadmium vaporizes, forms fine particles suspended in the air, and can poison anyone who breathes in the particles. As a result, it is often preferable to discard known cadmium-bearing scrap metal, treating it as hazardous waste, rather than to attempt to reuse it.
Like zinc and mercury, cadmium is almost inert to pure water but readily attacked by some dissolved species, particularly those which can form stable, water-soluble complexes with the cadmium ion. Ammonium compounds have a disproportionate effect because of the easy formation, high solubility and great stability of the cadmium tetrammine ion, formed by the reaction Cd.sup.2+ +4NH.sub.3 .fwdarw.Cd(NH.sub.3).sub.4.sup.2+. Cadmium dissolution is accelerated if oxygen or another oxidizing agent is present. Since ammonium compounds and oxygen are naturally present in rain and in most groundwater, cadmium-beating scrap which is buffed or exposed to the weather will tend to release cadmium to the environment in the form of soluble tetrammine salts. Like the metal and its uncomplexed ions, cadmium tetrammine salts are highly toxic. Hence, most cadmium-beating scrap requires special handling and disposal as toxic waste.
A special problem arises when waste is above the T.C.L.P. limit for a characteristic waste and is also radioactive. Such material is termed "mixed waste." Disposal of mixed waste is extremely difficult because of the conflicting regulations governing toxic and radioactive wastes. Much nuclear scrap, such as used reactor control and safety rods, must be handled as mixed waste because it contains not only cadmium but also residual amounts of radioactivity from neutron irradiation.
A number of processes are available for recovering metals, including cadmium, from acid solutions. Frankenfeld, et al. (U.S. Pat. No. 5,068,094) treat Cd-containing wet process phosphoric acid with a solution containing an amine or quaternary ammonium salt in an organic solvent, preferably a solution containing anionic chlorocomplexes of one or several polyvalent metals different from Cd. The Cd ions displace the polyvalent metal ions from the chlorocomplex salts, and may be recovered in the form of CdCl.sub.2 crystals. Rastas, et al. (U.S. Pat. No. 4,383,979) remove Zn, Cu and Cd from ferrites by leaching with sulfuric acid to produce ferritic solids and a sulfate solution of the nonferrous metals, and separating the solids from the sulfate solution. The invention is directed at optimizing the particle size of the solids. Calbeck (U.S. Pat. No. 4,133,865) prepares metallic sulfates by mixing the corresponding sulfide ores (ZnS, CuS, CdS, FeS) with ammonium sulfate and heating in a nonoxidizing atmosphere to convert the mixture to the metallic sulfate, ammonia, sulfur and sulfur dioxide.
Reinhardt, et al. (U.S. Pat. No. 4,053,553) recover Cd from nickel-cadmium battery waste by a process that includes the steps of leaching the waste with an ammoniacal carbonate solution to form a solution containing Cd, Ni, and Co(II) ammine complexes, and precipitating the Cd as a carbonate by removing ammonia from the solution. Hadzeriga (U.S. Pat. No. 3,853,941), Schulte-Schrepping, et al. (U.S. Pat. No. 3,721,729), George (U.S. Pat. No. 3,258,307) and Waring (U.S. Pat. No. 1,780,323) precipitate CdCO.sub.3 from an aqueous solution by adding ammonium carbonate to the solution; Schaufelberger (U.S. Pat. No. 2,837,406) adds ammonia and an ammonium salt to a leach liquor to form a soluble metal polyammine salt complex, recovers the complex, and calcines to recover the metal oxide. Thwaites (U.S. Pat. No. 921,312) forms CdS by adding finely-powdered ZnO to iron-sulfate-containing "pyrites liquor." The ZnO reacts with the sulfates to form iron oxides and ZnSO.sub.4. The precipitate is removed by filtration, and a soluble sulfide or sulfureted hydrogen is added to the remaining liquor to precipitate CdS. However, none of these processes is useful for recovering cadmium from cadmium-bearing scrap metals.
There is a need for a safe, effective process for recovering cadmium from cadmium-bearing waste metals, preferably without attacking any other metals present. Such a process would permit easier waste disposal, safer recycling of used materials, and the separation of much "mixed" nuclear waste into separate fractions which could then be handled according to well-established and nonconflicting regulations. Preferably, such a process would turn the cadmium into a stable, compact and insoluble form which either could be reused or, by meeting T.C.L.P. limits, could be discarded as nonhazardous. The stabilized cadmium product should be free of impurities, especially those from nuclear scrap which could be reasonably expected to carry radioactivity.